Field of the Invention
The invention relates to a conductor winding configuration for a large electrical machine. The conductor winding configuration can be impregnated with a filler and has a supporting body and at least one winding element which contains at least one electrical conductor which is sheathed by insulation which, for its part, is surrounded by a protective layer.
International Patent Applications WO 91/01059 A1 and WO 93/06646 A1 each disclose a conductor winding configuration for a large electrical machine having a ferromagnetic and electrically conductive supporting body and winding elements. The conductor winding configuration is intended in particular for use in a large electrical machine having an electrical rated consumption or emission of more than about 20 MVA. A winding element that is present in such a configuration has a base body with at least one electrically highly conductive metal wire or metal bar, in particular having a multiplicity of such metal bars. The base body is surrounded by insulation that is sheathed by an electrically semiconductive protective layer. The insulation is composed of a material which can be impregnated or which is impregnated with a filler, contains mica and is normally a strip wound on the base body and on a fine mica base. The filler is a synthetic resin, preferably a heat-curable epoxy resin system, for example a heat-curable mixture of an epoxy resin and an acid anhydrite. A base body of a winding element is configured in the form of a bar, in particular roughly straight with bent ends, and has a molded coil composed of wires wound on it. During the course of the so-called total impregnation method, the supporting body with the winding elements, which have wound around them unimpregnated insulation which can be impregnated, is initially completely assembled without any prior filler impregnation, and is subsequently impregnated as an entity. A low-viscosity filler is used for this purpose which fills every gap remaining in the configuration during impregnation, which is carried out under pressure. After curing, the conductor winding configuration is a single, compact solid body. A totally impregnated conductor winding configuration ensures excellent electrical characteristics, in particular high dielectric quality. Furthermore, the cured filler can contribute to the fixing of the winding elements in the slots, so that the requirement for slot sealing means is, under some circumstances, considerably reduced.
In the case of an impregnated winding element inserted in a slot in a supporting body, it is known from the above documents that temperature changes between normal ambient temperatures and considerably more than 100.degree. C. can occur even during the impregnation with filler and the subsequent curing of the filler, as well as during subsequent operation in a large electrical machine. Since the thermal coefficients of expansion of the supporting body and the filler differ from one another, thermal stresses can occur, which lead to the formation of cracks. Such cracks or gaps can, on the one hand, adversely affect the strength of the entire winding and, on the other hand, corona discharges can occur in the gaps during operation of the large electrical machine which not only adversely affect the quality of the dielectrics, but also lead to damage to the insulation over the course of time. In order to prevent such damage, International Patent Application WO 91/01059 A1 provides for a separating layer to be inserted between the slightly electrically conductive protective layer which sheaths the insulation and a further semiconductive protective layer. The separating layer results in the adhesion between the two slightly electrically conductive protective layers being reduced by the filler. The separating layer may be combined with the material of either the one or the other slightly conductive layer. The separating layer contains a separating means, which is not wetted by the filler. If synthetic resin is used as the filler, for example, the separating means is preferably an oily or fatty substance, polytetrafluoroethylene, a silicone compound or the like. The separating means is in this case added to a suitable medium, for example a textile or a film, it being possible for the latter to be made electrically semiconductive by suitable additives. Inserting the separating layer between two electrically semiconductive layers results in an electrically screened zone whose mechanical strength is less than that of the other zones of the winding element inserted in the slot. Cracks in the filler that are caused, for example, by thermal expansion and shrinkage processes preferably occur in this zone of reduced strength. The zone which deliberately has a tendency to crack formation (weak point) is well defined and no significant electrical potential gradients occur in it, so that spark formation in the cracks is virtually precluded, and there is no adverse effect on the electrical quality of the configuration.
A corona-discharge protection configuration for the stator winding of an electrical machine is disclosed in Published, Non-Prosecuted German Patent Application DE 42 19 064 A1. The corona-discharge protection configuration in this case includes two conductive layers located one above the other. The first layer being completely impregnated with an impregnation varnish to form main insulation, and adhering well to the main insulation. In contrast, the second layer is treated with an agent that repels the impregnation varnish, such that the second layer adheres neither to the first layer nor to the slot walls. This results in two conductive layers being produced which are parallel to one another and between which any separation resulting from thermal expansion can occur.
Published, Non-Prosecuted German Patent Application DE 36 36 008 A1 discloses an electrical machine having a stator winding which is produced using the total impregnation technique and has conductor elements which are insulated from one another and are combined to form winding bars. The conductor elements are disposed, surrounded by insulation, in slots in the laminated stator core, and are fixed in the radial direction by slot wedges. Any movement of the winding bars in the radial direction is prevented by the slot wedges being firmly bonded to the laminated stator core by an impregnation varnish. This solves the problem on which the German Patent Application DE 36 36 008 A1 is based, and which consists in that major expansion differences between the main insulation and the stator laminates occur when the electrical machine is heated or cooled, which expansion differences cannot be absorbed by the bonding points, so that local cracks and holes occur. In the case of high-rated machines, current forces which act mainly in the radial direction also occur in the conductors and in the slots in the laminated stack, so that, after initial loosening, these laminates can vibrate at twice the mains frequency. This could result in mechanical destruction of the main insulation and, finally, in failure of the electrical machine. The winding bars are surrounded on all sides by a sliding coating that, after impregnation and curing of the impregnation varnish introduced during the total impregnation method, has no adhesive connection to the main insulation. The sliding coating is composed of felt or fabric in the form of strips or webs, which is impregnated with a resin which contains conductive fillers, for example graphite powder, in order to achieve the necessary electrical conductivity of the sliding coating. Corona discharges are prevented by outer or slot corona-discharge protection, which may have to be provided, and around which the sliding coating is disposed. The conductivity of the sliding coating is such that a surface resistance in the order of magnitude of 1 kohm to 25 kohm per unit area is achieved. The sliding coating is manufactured from a glass-fiber felt or fabric that is impregnated with silicone resin, to which the conductive fillers are added.
Since the impregnation varnish is manufactured from an epoxy resin or polyester resin, bonding with the main insulation should be prevented, because of the different impregnation resin base of the sliding coating.
As a result of the fact that no adhesion takes place between the sliding coating and the main insulation, the winding bars can expand freely in the axial direction in the slot.